ZABUL PROVINCE, Afghanistan — Three sets of eyes peer out of a massively armored U.S. truck rolling slowly down Highway 1.

From inside their reinforced cocoon — constructed layer upon layer with ways to protect the human cargo inside — three Arizona National Guard Army engineers scan highway edges. They look for signs of digging, suspicious debris or any other anomaly in the dirt that hints at a buried explosive.

The rate of Americans dying or becoming dismembered by improvised explosive devices (IEDs), for 10 years the tormentor of U.S. forces, has dropped sharply as coalition troops withdraw from the battlefield. But lives still depend on what soldiers see or don't see. It's an enduring legacy of the homemade bomb that has created more American casualties over a decade and two wars than any other weapon.

What someone didn't see in the dirt along this same highway just 12 weeks earlier was a buried IED weighing hundreds of pounds. It killed 1st Lt. Jason Togi, 24, of Pago Pago, American Samoa, and an Afghan interpreter on a similar convoy mission riding in the same type of RG-31 armored truck.

"There's certain catastrophic explosions that it does not matter if you're in some sort of titanium ball," said Col. William Ostlund, commander of U.S. troops in this province.

So in the cramped quarters of the RG-31 this day, amid the smell of beef jerky and the cases of Burn and Rip It energy drinks, there lurks in the recesses of every soldier's mind one thought. Spc. Kyle Esplin, 22, who waits tables in Tucson; Spc. Brody Crane, 24, a part-timer at a Bass Pro Shop in Mesa, Ariz.; and Sgt. 1st Class Ramon Gomez, 33, who has a 5-month-old son, Emilio, back in Tucson, know that everything in their world could end in a violent millisecond.

Somewhere between more than half to two-thirds of Americans killed or wounded in combat in the Iraq and Afghanistan wars have been victims of IEDs planted in the ground, in vehicles or buildings, or worn as suicide vests, or loaded into suicide vehicles, according to data from the Pentagon's Joint IED Defeat Organization or JIEDDO.

That's more than 3,100 dead and 33,000 wounded. Among the worst of the casualties are nearly 1,800 U.S. troops who have lost limbs in Iraq and Afghanistan, the vast majority from blasts, according to Army data.

Soldiers clean vehicle windows as they prepare for the mission. Up to two-thirds of Americans killed or wounded in combat in the Iraq and Afghanistan Wars have been victims of IEDs planted in the ground, in vehicles or buildings, or worn as suicide vests or loaded into suicide vehicles, according to data from the Pentagon's Joint IED Defeat Organization. Jack Gruber, USA TODAY

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When one of the first Americans serving in Iraq, 25-year-old Pfc. Jeremiah Smith, 25, of Odessa, Mo., died in an explosion under his vehicle in May 26, 2003, six weeks after the U.S. invasion ended, the military wasn't even sure what to call the thing that killed him.

The Defense Department inadvertently applied an oxymoron, saying he was "hit by unexploded ordnance." Officials couldn't possibly know at the time that this weapon — what would come to be called in military parlance an improvised explosive device, a term now in common usage by those in and out of uniform — would be the most destructive of two wars.

The terror of the weapon continues to this day. Even as American forces leave Afghanistan, small numbers of U.S. soldiers gamble their lives on bomb-ridden roads or pathways.

November marked 10 years since the first U.S. death in Afghanistan blamed, when it happened, on an IED: Sgt. Jay Blessing, an Army Ranger, killed when the "thin-skinned" or unarmored Humvee he was driving was hit by a buried bomb Nov. 14, 2003.

The military has since gone back to identify a few earlier cases that technically qualify as IED attacks, including the death of Navy SEAL Matthew Bourgeois, 35, of Tallahassee, from a land mine wired to homemade bombs near Kandahar on March 28, 2002.

The IED, made for as little as a few hundred dollars each and produced by the thousands yearly first in Iraq and then Afghanistan, has changed the arc of how America wages war and how military medicine cares for the wounded.

It is a considerable feat for a triggering device made of wood and wire. Displayed at an IED investigative office at Bagram Air Base, they look like junior high workshop projects.

The bombs radically affected how the American military could move around the war zone, creating a heavy reliance on helicopters and other aircraft in order to avoid roads, says Army Lt. Gen. John Johnson, JIEDDO director.

"They've caused us a lot of pain ... a lot of effort and a lot of treasure," Johnson says.

Hundreds of millions in research dollars have been spent on understanding, identifying and treating the twin invisible maladies most often associated with these bombs: traumatic brain injury and post-traumatic stress disorder. Military and private researchers estimate the number of uniformed victims in the hundreds of thousands.

The IED has given rise to a multibillion-dollar industry in vehicle and body armor, robots, ground-penetrating radar, surveillance, electrical jamming, counterintelligence, computer analysis and computerized prostheses.

The Government Accountability Office says it's impossible to estimate the total U.S. cost of fighting the bombs over two wars. But the Pentagon has spent at least $75 billion on armored vehicles and tools for defeating the weapons.

In 2007, when American troops were losing limbs from blasts about every other day on average, the word IED — a military acronym for "improvised explosive device" — was so widely used it formally entered the American lexicon, accepted into Merriam-Webster's Collegiate Dictionary.

Fours year later, at the height of the Afghanistan War, the pace of U.S. troops suffering major amputations increased to one every 36 hours.

SURVIVING A BLAST

They call it "going boom." The first time for Spec. Leif Skoog, 23, a roofer back in Phoenix, was Oct. 3. He and Crane were in an RG-31 that was pushing an 8,000-pound roller in front of the vehicle, a device designed to detonate anything buried before the truck passes over it.

That's exactly what happened. The roller was destroyed, but the RG-31 survived. For those inside, there was the shock of the explosion, painful ear pressure, air made black with billowing dirt and dust, and a chemical smell that burned the nostrils.

Skoog, closer to the blast in the driver's seat, was stunned and disoriented. "It's not a physical wound," he recalls. "It's more like something doesn't feel right."

He showed signs of a mild traumatic brain injury from blast exposure, what scientists call the signature wound of the Iraq and Afghanistan wars. With dizziness, headaches and minor concentration problems, Skoog was kept out of combat for two weeks.

Understanding the frequency of these wounds in a war where body and vehicle armor block shrapnel but the IED blast wave can still damage the brain was one of the hardest lessons learned by military medicine from modern wars.

"It was like a slow awakening for everybody," says Chris Macedonia, a doctor and former adviser to the chairman of the Joint Chiefs of Staff, now-retired admiral Michael Mullen. "There were phenomena that were happening, particularly related to IEDs, that just didn't match what the education and teaching were before."

Doctors found that repeated mild brain injuries from blasts — without allowing the brain time to heal — can cause permanent neurological damage, risking later onset of Alzheimer's, Parkinson's or the even more debilitating chronic traumatic encephalopathy.

A RAND Corp. report estimated in 2008 that perhaps 320,000 troops, even at that early date, had suffered concussions or mild brain injuries, mostly from blast exposure. Pentagon officials the next year put the number at 360,000.

Most were never diagnosed when the wounds occurred and sent right back into combat, and no one knows the accurate number today, says Terri Tanielian, a RAND senior research analyst.

Not until 2010, nine years into the fighting, did three military leaders — Mullen; retired general Peter Chiarelli, then-Army vice chief of staff; and Marine Commandant James Amos — push through sweeping battlefield treatment changes requiring blast-exposed troops to be pulled from combat until, as with Skoog, symptoms go away.

"It took us a long, long time," Macedonia says. "Too long."

MORE PROTECTION FOR TROOPS

As early as 2003, U.S. field commanders in Iraq began demanding for their troops something other than the boxy Humvees that were being ripped apart by this new weapon.

Soldiers and Marines had taken it upon themselves to add so-called Hillbilly armor to their vehicles or pile sandbags on the floorboards.

The Pentagon initially rushed kits to retrofit Humvees with better protection in 2003 and 2004. But the trucks remained vulnerable because of their "flat bottom, low weight, low ground clearance and aluminum body," a Pentagon inspector general report found.

A Bush administration certain the Iraq War would be short-lived failed to supply large numbers of new Mine Resistant Ambush Protected (MRAP) trucks like the RG-31 until 2007. In the meantime, more than 1,400 U.S. troops died in IED blasts and 13,000 were wounded, according to JIEDDO data.

It was a USA TODAY story about the effectiveness of a limited number of MRAPs in saving the lives of Marines that led then-Defense secretary Robert Gates to order a crash program to churn out 27,000 of the trucks, including an all-terrain version for Afghanistan.

About $2 billion was spent training troops in dealing with IEDs, with elaborate exercises involving actors, explosions and fake gore set up in the California desert at Fort Irwin to mimic combat in Iraq and Afghanistan.

Another $7 billion went for intelligence operations to dismantle networks financing, producing and placing IEDs.

Today in the twilight of American involvement in Afghanistan, commanders are cutting the chance of death by IED even further.

Missions to clear roads, among the last going "outside the wire," are pulling back to paved highways where burying bombs is harder. Clearance convoys are shadowed by Apache attack helicopters. Night missions, peripheral lights ablaze, look like roving football stadiums.

From inside their armored vehicles, they can remotely inspect and probe suspicious ground with long metal arms. They can deploy robots big and small. They have electronic jammers, ground-penetrating radar and giant IED-uncovering rakes.

"There's been some crazy devices that we're not even going to use," Spc. Crane says about the many inventions provided to them.

An Obama administration eager to put the IED chapter behind it has pledged to avoid long-term operations where the bombs are a threat. And as troops come home, the Pentagon is gradually turning much of its MRAP fleet in Afghanistan into scrap.

When Arizona National Guard engineers were clearing roads in Zabul province on Nov. 17, Staff Sgt. Alex Viola, 29, of Keller, Texas — a Green Beret on foot patrol in nearby Kandahar province advising Afghan troops — stepped on an IED and was killed.Four days earlier, another Green Beret — Staff Sgt.Richard Vazquez, 28, of Seguin, Texas — was killed by an IED on the same type of mission.

Afghan civilians and troops are now suffering the brunt of IED attacks. But the bombs still kill and maim Americans, says Navy Capt. Dan Gramins, a vascular surgeon at a U.S. hospital in Kandahar, where a trickle of U.S. casualties from IEDs arrive each month.

It was at this facility that Army Cpl. Joshua Hargis, a Ranger who lost both legs to an IED blast on Oct. 6, delivered the "salute seen around the world" from his hospital bed after receiving a Purple Heart that later went viral.

If a soldier devastated by an IED can survive to reach a hospital, military doctors have become extremely proficient at saving them. "We're good at keeping them from dying," says Gramins. "But I think as long as we're fighting IEDs, we're going to continue to have multiple amputees."

The Arizona National Guard Army engineers will finish their 10-month deployment before Christmas. So there's a giddiness when a mission is over, as soldiers tumble out of their armored containers and bear-hug one another with exaggerated glee.

Inside the wire, they know where they step is safe.

"All of us know that when you go outside the wire," says Spec. Crane, "the threat is there."